Studies described in this proposal are directed in part toward determining the ionization behavior of Asp-158, His 159 and Cys-25 at the active site of papain and determining the roles of these amino acyl residues in catalysis. Ionizations are being studied by potentiometric difference titrations, nmr and fluorescence titrations of papain and papain derivatives. It is anticipated that the results of this research will be relevant to other metabolically important enzymes which utilize a thiolate anion as a nucleophile. In a study of another proteolytic enzyme system amino acid replacements in dysfibrogenemias are being determined in an effort to characterize the structural determinants of the physiological activity of fibrinogen. Studies described in this proposal are also concerned with determining the role in catalysis of amino acyl residues in the action of the vitamin B6-requiring enzyme D-serine dehydratase.